Windows Vista provides option to add additional clocks to your PC. This can be done using the Date Time settings available in Control Panel section.
As shown above the Date Time dialog contains an extra tab, Additional Clocks which would allow you to define the extra clocks and the Time Zone for the clocks. This would be very helpful if you are working in a different Time Zones. For example if you need to monitor three different time zones, then you need to set these time zones for each additional clocks respectively. And if you have selected show this clock option then these clocks would be displayed in the when you click on the Date Time in the Task Bar.